In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
Sintered bodies like cutting tool inserts etc. are usually made from materials containing cemented carbides or titanium based carbides or carbonitride alloys.
Titanium based carbides or carbonitride alloys are usually called cermets and contain one or more hard constituents such as carbides or carbonitrides of, e.g., tungsten, titanium, tantalum, niobium etc. together with a binder phase, which makes it possible to achieve attractive properties with regards to hardness and toughness. Cermets are useful in many applications, for instance in metal cutting tools, in wear parts etc. The properties can be adapted for a certain application by changing composition and grain size. The sintered bodies are made by techniques common in powder metallurgy like milling, granulation, compaction and sintering. The binder phase in cermets is usually Co, Fe or Ni or mixtures thereof.
The first cermet materials developed were TiC-based. In the eighties carbonitride-based cermets were introduced and a large part of the cermet materials developed since then are carbonitride-based.
For conventional cemented carbide, i.e., WC—Co based, fine grained particles after sintering can be obtained by adding chromium. However, when adding chromium to a carbonitride based cermet, no or little effect on the grain size can be seen.
CN 1865477 A discloses a guide roll, spool or valve seat of a TiC—WC based alloy comprising 30-60 wt % TiC, 15-55 wt % WC, 0-3 wt % Ta, 0-3 wt % Cr and 10-30 wt % of a binder phase being Co and Ni.
U.S. Pat. No. 7,217,390 describes a method of making an ultra-fine TiC-based cermet by mechano-chemical synthesis, e.g., high-energy ball-milling of powders of Ti, transition metal (M), Co and/or Ni powders and carbon powders. Alternatively the Ti and transition metals can be added as carbides. The transition metal, M, can be at least one element of Mo, W, Nb, V or Cr. The high-energy ball-milling will form (Ti,M)C.
However, the high-energy ball-milling is a complicated process and it would be beneficial to be able to provide a fine-grained TiC-based cermet using conventional techniques.
In conventional TiC-based cermets, a large amount of the TiC has been dissolved and new Ti—W—C grains have been formed, which leads to uncontrolled Ti—W—C grain growth and uneven grain size and deterioration of properties like hardness.
Group V elements such as Nb, Ta and V and carbides thereof, are known as grain growth inhibitors for cemented carbides. However, adding e.g. NbC to Ti(C,N) based cermets does not decrease the grain size because the amount of TiN in the alloy is the dominating parameter in these alloys. Adding group V elements such as Nb, Ta and V and carbides thereof to these cermets increases the formation of softer rims surrounding the Ti(C,N) grains, resulting in a detrimental decrease of the hardness.
Adding carbides of group V elements, e.g., NbC, to cermets results in an increase in hot hardness and improvement of plastic deformation at higher cutting temperatures, however it also decreases wear resistance at lower cutting temperatures.
By the present disclosure, however, the total grain size is decreased by nucleating new cores (and rims with the same composition as the new cores) with smaller grain size than in the starting material, keeping the hardness unchanged.